The present embodiments relate to integration of imaging systems, specifically magnetic resonance (MR) imaging and positron emission tomography (PET) systems.
PET may be combined with another imaging modality in a multimodality system. Such multimodality imaging systems may have diagnostic and business value. Both PET/computed tomography (CT) and single photon emission computed tomography (SPECT)/CT multimodality imaging systems allow scans to be performed back-to-back or in a same coordinate system and with similar timing. The axial fields of view of the individual modalities are as close together as possible in order to minimize the impact of patient motion and increase spatial correlation of the respective data sets.
Another hybrid example is a brain scan PET system integrated with a magnetic resonance (MR) system. In order for the MR and PET fields of view to overlap, the PET detectors are placed as an insert in front of the body coil. The MR body coil is used to excite the molecules of the patient by delivering an RF burst. The MR switches into a receive mode, after delivery of the RF burst, and detects RF signals emitted from the patient. The signal-to-noise ratio of the MR received signal is an important aspect of MR imaging. The signal-to-noise ratio is important enough that a typical MR system is enclosed in a radio frequency (RF) cabin that suppresses RF signals, such as by 100 dB, for both external signals entering the RF cabin and internal signals exiting the RF cabin.
Electromagnetic interference (EMI) and electromagnetic compatibility (EMC) between the MR and PET subsystems is one of the dominant technical challenges facing MR/PET integration. The MR subsystem is extremely sensitive to any RF emissions from the PET subsystem, near the hydrogen spin frequency (e.g., roughly 123 MHz+/−500 KHz for a 3 Tesla system). Likewise, the PET front end is extremely vulnerable to the RF emissions from the MR subsystem. Coincidence windows of 4-10 nS are typical of non-time-of-flight PET scanners, which corresponds with a PET signal chain stable to 100 pS. For the brain scan PET/MR system, the detector signals are routed out of the RF cabin to avoid EMI and EMC issues with the MR subsystem. Outside the cabin, the signals are amplified and filtered. However, the length of the cabling may have detrimental effects on signal integrity and timing precision for the PET subsystem. The volume and weight of the cabling may introduce other complications, including performance limitations through restrictions on the number of PET detectors in the integrated system.